Field of the Invention
U.S. natural gas production is increasing and the price of natural gas is currently lower than the price of gasoline or diesel fuel, leading to increasing interest in natural gas-based fuels for vehicles. The most common type of natural gas vehicle operates on compressed natural gas (CNG), but there is also an interest in liquefied natural gas (LNG) as a vehicle fuel, especially for commercial trucks because LNG, which is natural gas super-cooled to its liquid form, has a much higher energy density than CNG.
Description of the Related Art
At CNG refueling stations, the natural gas is typically taken from the local gas utility's line at low pressure, compressed to around 3,600 pounds per square inch gauge (“psig”), and then stored in a vehicle's storage tanks at high pressure. For example, at a “fast-fill” CNG station, the combination of a relatively large compressor coupled with a high-pressure storage tank system fills the vehicle's storage tanks in about the same amount of time it takes to fuel a comparable petroleum vehicle. A typical fast-fill CNG station is shown in FIG. 1. Some of the major components of the fast-fill CNG station include an inlet gas line 10 (from a utility company); a dryer 15 to reduce the moisture content of the natural gas; and a natural gas compressor 20. One example of a natural gas compressor 20 is an Ingersoll Rand compressor package, which includes a compressor, an electric motor, a motor starter, a cooler, and controls. The compressor package will increase the pressure of the natural gas in the inlet gas line 10 from about 5 pounds per square inch gauge (“psig”) up to about 5,000 psig. At least one storage vessel 25 is capable of holding natural gas at about 5,000 psig and supplying the natural gas to a gas dispenser 30 for dispensing to a vehicle's storage tanks.
LNG stations are structurally similar to gasoline/diesel stations, because they both deliver a liquid fuel. FIG. 2 illustrates some typical components of an LNG station, including a storage tank 50, a pump 55 for transmitting the liquid fuel from the storage tank 50, some type of a card reader 60 for charging a customer for the liquid fuel, and a dispenser 65 to carry the liquid fuel to a vehicle 70. In the mobile fueling arrangement shown in FIG. 2, LNG is delivered by a tanker truck that contains metering and dispensing equipment onboard to fill the storage tank 50.
In addition to expenses related to construction ($1 to $4 million, according to the Energy Information Administration), a fueling site like the ones shown in FIGS. 1 and 2 require at least one storage tank as well as pumps/compressors. For this reason, there is interest in converting gasoline/diesel stations to those that can supply CNG/LNG. Current attempts to retro-fit existing stations have envisioned setting aside surface area for the tanks, compressors, pumps, and related equipment or even excavating the gasoline/diesel tanks and replacing them with tanks suitable for natural gas. These solutions are expensive and create a substantial change of the footprint of the filling station.
There is a need therefore, for a simple and efficient arrangement to convert or retro-fit a gasoline/diesel station into one that can also provide CNG and/or LNG.